/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/atomic.h"
#include "libc/calls/sched-sysv.internal.h"
#include "libc/cosmo.h"
#include "libc/errno.h"
#include "libc/intrin/atomic.h"
#include "libc/thread/thread.h"

enum {
  INIT,
  FINISHED,
  CALLING,
};

dontinline static errno_t cosmo_once_impl(atomic_uint *once, void init(void)) {
  unsigned word;
  for (;;)
    switch ((word = atomic_load_explicit(once, memory_order_acquire))) {
      case INIT:
        if (atomic_compare_exchange_strong_explicit(once, &word, CALLING,
                                                    memory_order_acquire,
                                                    memory_order_relaxed)) {
          init();
          atomic_store_explicit(once, FINISHED, memory_order_release);
          return 0;
        }
        // fallthrough
      case CALLING:
        pthread_yield_np();
        continue;
      case FINISHED:
        return 0;
      default:
        return EINVAL;
    }
}

/**
 * Ensures initialization function is called exactly once.
 *
 * This is the same as pthread_once() except that it always uses a tiny
 * spinlock implementation, and won't do heavyweight calls. It's needed
 * since this function is an upstream dependency of both pthread_once()
 * and nsync_once(). Most code should favor calling those functions. If
 * you use this, your callback can't throw exceptions and it should not
 * call pthread_exit() or be asynchronously canceled; it will deadlock.
 *
 * This implementation is process shared.
 *
 * @return 0 on success, or errno on error
 */
errno_t cosmo_once(atomic_uint *once, void init(void)) {
  if (atomic_load_explicit(once, memory_order_acquire) == FINISHED)
    return 0;
  return cosmo_once_impl(once, init);
}
